I thought I'd phrase my answer in the form of a question...no wait, that isn't exactly right. Okay, let me just say that this is one short little post that falls in the category of "It's the Little Things You Love" about SharePoint 2013. Today's feature call out is about something that has been asked for since SharePoint 2007 and has finally found it's way into the product. The answer to the question above is, "whatever content database you want!". :-)
The good news here is that when you look at the New-SPSite cmdlet now, you have a parameter called -ContentDatabase. You can pass in the name of a content database and the site collection will be created in that database. No more horsing around with putting content DBs to offline mode, or changing the max. number of sites just to try and force SharePoint to pick the right one. You now have the power my friends - enjoy!
NOTE: Blair B. was good enough to point out to me that this feature actually made it's way into SharePoint 2010. At first I was going to just pull this posting down, but after doing a number of web searches it seemed that virtually no one knows about it, or if they do they haven't been talking about it. So, sorry I missed this addition earlier, but I'm going to go ahead and leave this post up so that hopefully we can spread the word about it's availability. Thanks for that update Blair.